Provided by: libur-perl_0.470+ds-1_all bug


       UR::Role - Roles in UR, an alternative to inheritance


         package My::Role;
         role My::Role {
             id_by => [
                 role_id_property => { is => 'String' },
             has => [
                 role_property => { is => 'String' },
                 another_prop  => { is => 'Integer' },
             requires => ['class_method'],
             excludes => ['Bad::Role'],
         sub role_method { ... }

         package My::Class;
         class My::Class {
             has => [
                 class_property => { is => 'Integer ' },
             roles => ['My::Role'],
         sub class_method { ... }

         my $obj = My::Class->new();
         $obj->does('My::Role');  # true


       Roles are used to encapsulate a piece of behavior to be used in other classes.  They have
       properties and methods that get melded into any class that composes them.  A Role can
       require any composing class to implement a list of methods or properties.

       Roles are not classes.  They can not be instantiated or inherited from.  They are composed
       into a class by listing their names in the "roles" attribute of a class definition.

   Defining a Role
       Roles are defined with the "role" keyword.  Their definition looks very similar to a class
       definition as described in UR::Object::Type::Initializer.  In particular, Roles have a
       "has" section to define properties, and accept many class-meta attributes such as
       'id_generator', 'valid_signals', and 'doc'.

       Roles may implement operator overloading via the 'use overload' mechanism.

       Roles also have unique attributes to declare restrictions on their use.

           A listref of property and method names that must appear in any class composing the
           Role.  Properties and methods defined in other roles or parent classes can satisfy a

           A listref of Role names that may not be composed together with this Role.  This is
           useful to declare incompatibilities between roles.

   Composing a Role
       Compose one or more Roles into a class using the 'roles' attribute in a class definition.

         class My::Class {
             roles => ['My::Role', 'Other::Role'],
             is => ['Parent::Class'],
             has => ['prop_a','prop_b'],

       Properties and meta-attributes from the Roles get copied into the composing class.
       Subroutines defined in the Roles' namespaces are imported into the class's namespace.
       Operator overloads defined in the Roles are applied to the class.

       Property and meta-attribute conflicts

       An exception is thrown if multiple Roles are composed together that define the same
       property, even if the composing class defines the same property in an attempt to override

       A class may declare a property with the same name that a role also declares.  The
       definition in the class overrides whatever appears in the role.  An exception is thrown if
       a role declares an ID property in the 'id_by' section and the consuming class redeclares
       it in the 'has' section as a normal property.

       Method conflicts

       An exception is thrown if multiple Roles are composed together that define the same
       subroutine, or if the composing class (or any of its parent classes) defines the same
       subroutine as any of the roles.

       If the class wants to override a subroutine defined in one of its roles, the override must
       be declared with the "Overrides" attribute.

         sub overridden_method : Overrides(My::Role, Other::Role) { ... }

       All the conflicting role names must be listed in the override, separated by commas.  The
       class will probably implement whatever behavior is required, maybe by calling one role's
       method or the other, both methods, neither, or anything else.

       To call a function in a role, the function's fully qualified name, including the role's
       package, must be used.

       Overload conflicts

       Like with method conflicts, an exception is thrown if multiple Roles are composed together
       that overload the same operator unless the composing class also overloads that same

       An exception is also thrown if composed roles define incompatible 'fallback' behavior.  If
       a role does not specify 'fallback', or explicitly sets it to "undef", it is compatible
       with other values.  A Role that sets its 'fallback' value to true or false is only
       compatible with other roles' values of undef or the same true or false value.

       Each time a Role is composed into a class, its "__import__()" method is called.
       "__import__()" is passed two arguments:

       ·   The name of the role

       ·   The class metadata object composing the role.

       This happens after the class is completely constructed.

   Parameterized Roles
       Scalar variables with the "RoleParam" attribute are designated as role params.  Values can
       be supplied when a role composes the role as a means to provide more flexibility and
       genericity for a role.

         package ObjectDisplayer;
         use ProjectNamespace;

         our $target_type : RoleParam(target_type);
         role ObjectDisplayer {
             has => [
                 target_object => { is => $target_type },

         package ShowCars;
         class ShowCars {
             roles => [ ObjectDisplayer->create(target_type => 'Car' ],

       When the role is composed, the call to "create()" in the class definition creates a
       UR::Role::Instance to represent the ObjectDisplayer role being composed into the ShowCars
       class with the params "{ target_type =" 'car' }>.  Values for the role param values in the
       role definition are swapped out with the provided values as the role's properties are
       composed into the class.

       At run-time, these role param variables are tied with the UR::Role::Param class.  Its
       "FETCH" method searches the call stack for the first function whose invocant composes the
       role where the variable's value is being fetched from.  The proper param value is

       An exception is thrown if a class composes a role and either provides unknown role params
       or omits values for existing params.

   Method Modifiers
       Roles can hook into methods defined in consuming classes by using the "before", "after"
       and "around" method modifiers.

         use UR;
         package RoleWithModifiers;
         use UR::Role qw(before after around);
         role RoleWithModifiers { };
         before 'do_something' => sub {
             my($self, @params) = @_;
             print "Calling do_something with params ",join(',',@params),"\n";
         after 'do_something' => sub {
             my($rv, $self, @params) = @_;
             print "Result from do_something: $rv\n";
         around 'do_something' => sub {
             my($orig, $self, @params) = @_;
             print "Wrapped call to do_something params ",join(',',@params),"\n";
             my $rv = $self->$orig(@params);
             print "The wrapped call to do_something returned $rv\n";
             return 123;

         package ClassUsingRole;
         class ClassUsingRole { roles => 'RoleWithModifiers' };
         sub do_something {
             print "In original do_something\n";
             return 'abc';

         my $rv = ClassUsingRole->create()->do_something();
         print "The call to do_something returned $rv\n";

       Running this code will generate the following output:

         Wrapped call to do_something params
         Calling do_something with params
         In original do_something
         Result from do_something: abc
         The wrapped call to do_something returned abc
         The call to do_something returned 123

       Method modifiers are applied in the order they appear in the role's implementation.

           A "before" modifier runs before the named method. It receives all the arguments and
           "wantarray" context as the original method call.  It cannot affect the parameters to
           the original method call, and its return value is ignored.

       after($rv, @params)
           The first argument to an "after" modifier is the return value of the original method
           call, the remaining arguments and "wantarray" context are the same as the original
           method call.  If the original method was called in list context, then $rv will be an
           arrayref containing the list of return values.  This modifier's return value is

       around($orig, @params)
           An "around" modifier is run in place of the original method, and receives a coderef of
           the original method as its first argument.  Around modifiers can munge arguments and
           return values, and control when and whether the original method is called.


       UR, UR::Object::Type::Initializer, UR::Role::Instance, UR::Role::Param